The present invention relates generally to delivery systems and methods for deploying medical devices and, more particularly, to delivery systems and methods for their use to accurately deploy medical devices, such as a stent, a vascular stent-graft and the like, in a body vessel of a patient for the treatment of stenosis, aortic aneurysms and other afflictions which may strike body vessels. The present invention also can be used to deliver medical devices for arthroscopic surgery and other surgical procedures.
Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there through. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty, percutaneous transluminal angioplasty, or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis. Stents, or stent like devices, are often used as the support and mounting structure for implantable vascular grafts which can be used to create an artificial conduit to bypass the diseased portion of the vasculature, such as an abdominal aortic aneurism.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self expanding stents inserted into a compressed state for deployment into a body lumen. One of the difficulties encountered in using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery through the often tortuous paths of the body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self expanding stent formed from shape memory metals or superelastic nickel titanium alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter, or when a restraining sheath which holds the compressed stent in its delivery position is retracted to expose the stent.
Some prior art stent delivery systems for delivery and implanting self-expanding stents include an inner member upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner member to “uncover” the compressed stent, allowing the stent to move to its expanded condition. Some delivery systems utilize a “push pull” type technique in which the outer sheath is retracted while the inner member is pushed forward. Another common delivery system utilizes a simple pull-back delivery system in which the self-expanding stent is maintained in its compressed position by an outer sheath. Once the mounted stent has been moved to the desired treatment location, the outer sheath is pulled back via a deployment handle located at a remote position outside of the patient, which uncovers the stent to allow it to self expand within the patient. Still other delivery systems use an actuating wire attached to the outer sheath. When the actuating wire is pulled to retract the outer sheath and deploy the stent, the inner member must remain stationary, preventing the stent from moving axially within the body vessel.
Controlled deployment of the stent can be a desirable feature in various applications. This can be particularly true when attempting to deploy a self-expanding stent which may tend to spring forward as the sheath is being removed. Moreover, stents and stent-grafts are being made in longer lengths for implantation in peripheral vessels, such as the arteries of the leg, to treat conditions such as Peripheral Arterial Disease (PAD). Such longer stents often require additional deployment time in contrast to shorter stents which are implanted in the coronary arteries. When a long stent is deployed from a catheter utilizing a retractable outer sheath, the initial deployment force is high since static friction between the stent and the outer sheath, along with the remaining catheter components, needs to be overcome. Static friction, sometimes referred to as “striction,” between the retraining sheath and the stent can pose a problem since a high, initial deployment force must be applied to the actuating mechanism of the delivery system in order to commence retraction of the sheath to uncover the stent. Therefore, it is important that the delivery system provide at least some amount of “mechanical advantage” when the system's actuating mechanism is initially engaged so that the physician is not struggling to get the retraction started. Mechanical advantage refers to the ratio of the output force of an actuator to the input force and is achieved when the ratio is greater than one. The larger the mechanical advantage, the less force is needed to initiate movement of the actuating mechanism. Moreover, the delivery system usually requires a slower and more controlled deployment rate when initially retracting the outer sheath to initiate deployment of the stent in the body vessel. This is to ensure that the stent is placed accurately in the body vessel.
After the stent is somewhat deployed, the amount of deployment force needed to retract the remaining outer sheath is reduced since dynamic frictional forces are typically lower than static frictional forces. After the sheath starts to move, the deployment force needed to continue retraction drops off quickly to less than about 60% of then initial deployment force. Moreover, once the distal-most portion of the stent has made some wall apposition with the body vessel, it is advantageous to quickly deploy the remainder of the stent. Therefore, it may be desirable to employ a delivery system which provides additional delivery speed once static friction is overcome, especially when the length of the stent or graft is quite long. Accordingly, an ideal delivery system should reduce the amount of actuating motions imparted by the user once retraction has begun. Accordingly, it has been found to be desirable to have a delivery system which provides sufficient control and mechanical advantage for initially deploying the medical device which then translates to increased delivery speed once retraction has begun in order to assist the physician in quickly and accurately deploying the medical device.
The present invention disclosed herein satisfies these and other needs.